Studies on carcinoembryonic antigen (CEA) in rats were continued using transplantable colonic tumor RCA-1 that was originally induced by 1,2-dimethylhydrazine. The rat CEA was shown to have similar physicochemical properties to human CEA but these two antigens did not show any serological cross-reactions. Investigations on the resistance to RCA-1 of rats immunized with rat CEA in complete Freund's adjuvant were extensively pursued, and this study has now encompassed close to 100 rats. Definite resistance to the tumor in the immunized animals was noted; at the time when only 19% of immunized animals showed growth of RCA-1, over 90% of immunized animals accepted a colonic tumor RCA-2, which lacks CEA, and a similar proportion of nonimmunized animals would accept either RCA-1 or RCA-2. Further animal studies are still needed before human trials can be considered. Considerable progress was made in studies on heterophile antigens and antibodies. From these studies, the classification of medically important heterophile antibodies emerged that is based on their reaction with guinea pig kidney tissue. Further studies on the occurrence of Paul-Bunnel antigens showed these antigens in sera of 10 to 20% of patients with syphilis and lepromatous leprosy. On the other hand, Hanganutziu-Deicher antigen was found with high frequencies exceeding 25% in malignancies, Chediak-Higashi syndrome, and Kawasaki disease. Some investigations were also devoted to the interesting heterophile system of Newcastle disease virus. Antibodies to this virus were detected in over 53% of patients with infectious mononucleosis and 20 to 30% of patients with lepromatous leprosy, SLE, and syphilis as well as renal graft recipients. Detection of thermostable, ethanol insoluble, organ-specific antigens was pursued to develop diagnostic tests for organ destruction by means of inhibition of enzymoimmunoassay. Brain-specific antigen could be detected in sera of 14 of 43 patients with cerebrovascular accident. The strength of reaction paralleled the size of the lesion. Application of this procedure for detection of malignancies in various organs, especially of metastatic lesions, is pursued. (AG)